Erg-margin deposits in the Lower Jurassic Moenave Formation and Wingate Sandstone, southern Utah

Abstract

The Moenave Formation (Dinosaur Canyon Member) and the closely associated Wingate Sandstone in southern Utah represent the southwestern erg margin of the Lower Jurassic Wingate erg. The erg-margin deposits, which extend across a 50- to 100-km-wide region, are composed of dune, eolian sand-sheet, mud-flanked eolian, sabkha, ephemeral-stream and lacustrine facies. The succession is divided into a basal erg-margin association (8-27 m) dominated by eolian facies and an overlying erg-margin association (as much as 90 m thick) composed mainly of fluvial and lacustrine facies. The sediments of the basal association are composed of three drying-upward or locally drying-wetting-upward sequences that can be traced from the erg margin 100 km slightly upwind (that is, toward the west-northwest) into the outer erg-margin zone. Sequences of the inner erg-margin zone contain dune facies in their upper parts, sequences in the intermediate erg-margin zone contain sand-sheet facies, and those in the outer erg-margin zone contain sabkha facies. The basal association thins markedly toward the west-northwest. Sediments of the overlying association contain up to six drying-upward or drying-wetting-upward sequences but are apparently only fully preserved in the outer erg-margin zone. Individual sequences show much lateral variation but characteristically contain lower, muddy, ephemeral-stream deposits and upper, sandy, ephemeral-stream deposits; sand-sheet deposits; or incipientdune deposits. The general time trend is interpreted as a gradual change toward a more humid climate. The stacked drying-upward or drying-wetting-upward sequences suggest that this long-term climatic change was characterized by a number of fluctuations between relatively arid and humid conditions. These climatic fluctuations apparently controlled both sediment yield to the basin and the depositional conditions in the erg-margin region. During arid intervals, ephemeral braided streams transported sandy bed-load into the erg-margin area, where eolian reworking took place. During humid intervals, sheet-floods and high-sinuosity rivers transported a more mud-rich sediment load into the basin, where eolian activity was minimal.